1. Field of the Invention
The invention relates in general to an alternating current light emitting device, and more particularly to an alternating current light emitting device capable of directly using an AC voltage source of mains and having high light emitting efficiency.
2. Description of the Related Art
A light emitting diode (LED) has the high endurance, the long lifetime, the light and handy property and the low power consumption and does not contain harmful substances, such as mercury, and thus becomes an extremely ideal light emitting device for the new generation of illumination. Recently, the invention of the blue LED solves the problem of electrostatic discharge (ESD) protection. In addition, the enhancement of the luminance of the LED enables the application field of the LED to grow continuously, and the LED has become the indispensable and important illumination tool in the modern life. For example, LEDs may be used as indicators, displays, the indoor/outdoor illumination and the vehicle illumination, and the cost of the LED has been greatly reduced.
FIG. 1A (Prior Art) shows waveforms of an input voltage and a current in a conventional diode light emitting device. A threshold voltage of each micro-diode only ranges from 2 to 5V, so multiple micro-diodes have to be connected to form a string so that the string can be used and powered by the mains provided by the electric power company. Thus, the equivalent threshold voltage of the string of micro-diodes may reach about 90V or higher. In other words, the current cannot flow through the micro-diodes until the input voltage provided by the AC voltage source is higher than 90V (about t=0.002 to 0.006 seconds) in the positive half cycle of the AC voltage source. Similarly, the current cannot flow through the micro-diodes until the input voltage provided by the AC voltage source is lower than −90V (about t=0.010 to 0.014 seconds) in the negative half cycle of the AC voltage source.
FIG. 1B (Prior Art) shows waveforms of the current and a light output of an AC LED module in the conventional diode light emitting device. As shown in FIG. 1B, when no current flows through the micro-diodes, no light is outputted. In other words, the micro-diodes cannot output the light until the input voltage provided by the AC voltage source is higher than the positive and negative threshold voltage (i.e., t=0.002 to 0.006 seconds, and about t=0.010 to 0.014 seconds).
In general, the power may be divided into the apparent power and the real power in calculation. The apparent power is the product of the voltage and the least mean square of the current in one cycle, while the real power is the average of the products of the voltages and the currents at many points in one cycle. Furthermore, the power factor is the ratio of the real power to the apparent power. Usually, the too-small power factor causes the loading of the electric apparatus and the electric power wastage. For example, Taiwan electric power company requests the power factor to be greater than 0.8.
As shown in FIGS. 1A and 1B, it is obtained that the power factor of the micro-diodes powered by the AC voltage source must be smaller than 1. Furthermore, when the overall threshold voltage is too high, the proportion of the micro-diodes, which do not emit light, is increased so that the flicker extent is increased. In addition, the frequency of the input voltage provided by the AC voltage source also influences the flicker extent of the micro-diode. When the frequency of the input voltage is too low, the flicker extent of the micro-diode is increased.